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HP 180 Series Inc. AN/USM-281

November 15, 2011

Here we have a 180 TR (fitted with long persistence CRT) over a 180A on the left and a AN/USM 281 (180F I think) on the right. These are excellent and accurate scopes based on a 100MHz mainframe having an internally illuminated graticule. A precision wide-band X amplifier is included in the mainframe while the Y plates connect directly to the Y plug in. Most of the issues that arise with these scopes are mechanical, aged solder joints, dirty switches, loose switch nuts and various screws. I do sometimes experience component failures, the local decoupling capacitors in the timebases are especially likely to fail. Occasionally a transistor will fail, the solution is to logically isolate the circuit that has failed and take transistors out and test them until the culprit is found. In general, all the Y plug-in faults I have had are mechanical while the time bases are more likely to suffer component failure. They represent the first generation of solid state after the vacuum tube era and date from circa 1970.

I have read that the 180 “T” series displays are recommended for use with the 8755B network analyser. They provide zero offset recorder outputs, and both positive and negative 5-volt retrace blanking inputs.

The Y plug ins I have are all dual channel general purpose 50MHz types with delay lines, the 1801A in the 180TR and 180A having a sensitivity of 5mv/cm to 20v/cm while the PL1186A in the 180F has a x5 switch that increases the sensitivity to 1mv/cm.

One of the 1801A units has power connectors that provide +15v and -12.6v for active probes, I must try to obtain at least one of these probes! This unit appears to be a very early example, the internal configuration is quite a bit different to the other unit though they both bear the same model designation. A rather confusing point is that there is a screened wire that connects from the interface plug (between the time-base and the Y unit) to – nowhere! I can find no evidence under a magnifying glass that it was ever connected and the locations on the plug end are not used on the time-base side of any units I possess. These locations (14 & 15) are not assigned in the manual either. It appears to be completely redundant yet, is clearly an original part of the wiring loom. If anybody can shed light on this apparent aberration, please let me know!

The time bases are all delaying types and rather easy to use though the delayed section seems to be prone to failure for some reason.

The 180A is fitted with type 1822A which has a range of rates of 1S/cm to 50nS/cm, expandable by x10 to a significantly fast 5nS/cm. It also has a variable hold-off feature that can be used to increase the hold-off time between sweeps at the very high speeds this unit is capable of, to prevent double triggering along with a RF stability control. The downside of these extra controls is that they spoil the access to the trigger level control, I don’t recommend this unit, the extra features seem of little value compared with the inconvenient way they are “stuffed in”.

The 180TR is fitted with type 1821A which has a range of rates of 1S/cm to 100nS/cm, expandable by x10 to 10nS/cm.

The 180F is fitted with type PL1187A which has a range of rates of 2S/cm to 100nS/cm, expandable by x10 to 10nS/cm.

I believe the PL prefix types are intended for the USM-281 naval application.

Here is a picture of the magnificent 12kv CRT (everybody should have at least one CRT on their dining table). Unfortunately, it does not have a nice recognisable part number, in my 180F manual it is listed as HP part 5083-9023 which I guess has the standard P31 phosphor:

The military version includes a fine grounded mesh over the screen, presumably to minimise RF emissions, not X rays, the civilian version does not appear to include this feature. Since it designed to be removable, I have removed it from my AN.USM unit. It impacts the clarity surprisingly little but it does have some impact. Hopefully I am not dosing myself with X radiation!

180A and 180F showing two waveforms of unrelated frequency:

Here is a picture showing the difference in the civilian (bottom) and military (top) casing. I prefer the civilian design, it is much easier to get the two-piece clams shells off:

This picture shows the inside of the units, military below, civilian above, the internal differences are minor, you may spot the extra shielding around some capacitors on the CRT control board. The Y amp plug in is visible at the bottom right of each unit:

Similar view of the right side, the visible boards are the X amp, in the middle and the power supply regulators at bottom right, the time-base plug in is visible on the bottom left of each unit. It is also possible to spot the shock mounts for the CRT neck in the lower unit, these are not fitted to the civilian unit:

The next job with these scopes is to get the X amplifier in all the units calibrated properly so that I can interchange the plug ins without loosing calibration.

Now I have done that, here is a picture showing mixed mode operation of the 1821A timebase, the delaying sweep is to the left (10mS/div) and the delayed sweep (0.1mS/div) is to the right, magnifying the ringing of a power transformer by a factor of 100.


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  1. David Allen permalink

    I have a HP AN/ USM-338 what is it worth???

    • I have no idea. In general, the worth of these instruments is for me, that they are extant, not how much money I could get for them.

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